Why are micrometer calipers preferred for measuring diameters?

[splafpt]

Homework Statement

Why are micrometer calipers used instead of vernier calipers in measuring diameters?

Homework Equations

N/A

The Attempt at a Solution

Micrometer calipers are more accurate than vernier calipers but I am guessing that there's a better reason behind the preference of micrometer calipers in measuring diameter.

 

[splafpt]

Why are micrometer calipers used instead of vernier calipers in measuring diameters?

 

[schip666!]

Compare the connecting tissue of a mic and caliper, such as the two in this picture:
http://www1.mscdirect.com/ProductImages/0631697A-11.jpg

A micrometer has a much more rigid bow of material between the two measuring points and thus doesn't deform as easily. It also has a spring knob which provides repeatable tensioning between the points, so at least the stretch is similar each time you make a measurement. Plus it's adjustable and usually comes with a standard for calibration. Most calipers are "ok" to .001" but mics easily go down to .0001".

BTW, "vernier" refers to the extra stretched scale on many kinds of instruments, micrometers included, which allow you to get another measure digit (or two) by "reading between the lines". Almost all slide calipers, like the ones in that photo, have a dial or are digital these days, probably because using a vernier is tricky to learn and takes longer to read.

 

[splafpt]

Compare the connecting tissue of a mic and caliper, such as the two in this picture:
http://www1.mscdirect.com/ProductImages/0631697A-11.jpg

I'm sorry but the link you provided is inaccessible. Can you give another one, please?

 

[2milehi]

With a micrometer you can measure to a ten-thousandth of an inch. Plus you can "feel" when you are measuring correctly.

With a vernier caliper you can measure to one-thousandth of an inch. It is a little tricky to pick out the "lined up" part on the vernier part:
http://www.craftsmanspace.com/images/stories/tools%20and%20machines/measuring%20tools/measuring_with_metric_vernier_caliper.gif

 

[splafpt]

With a micrometer you can measure to a ten-thousandth of an inch. Plus you can "feel" when you are measuring correctly.

With a vernier caliper you can measure to one-thousandth of an inch. It is a little tricky to pick out the "lined up" part on the vernier part:
http://www.craftsmanspace.com/images/stories/tools%20and%20machines/measuring%20tools/measuring_with_metric_vernier_caliper.gif

I'm sorry but I don't feel very comfortable with the explanation merely based on "feel." Can you provide a more rhetorical explanation?

 

[schip666!]

That one works for me this morning but may have been down overnight. Here's a better one anyway: http://www.gagewebsite.com/wp-content/uploads/2011/04/micrometer1.jpg

The middle caliper is a "vernier" style. You can see the vernier markings above and below the main measure scale. But the main point is that the construction of the micrometer is much more solid. You may think this is unnecessary -- and much of the time it is -- but I used to do a demo for my shop classes where I setup a 1" bar in a lathe chuck with a dial gauge like the one in the lower left of that photo. Then I could just push on the end of the bar with a single finger and show that it deflected by 5 mils without much force at all.

 

[Redbelly98]

With a caliper, you are physically pushing the two faces directly towards each other with your thumb. Not so with a micrometer, where the two faces are moved together by _____________ (what is it that you move with your fingers?).

It would seem that with those two methods of moving the faces, one gives a more accurate reading than the other.

 

[Studiot]

Firstly we are doing splafpt a disservice here in allowing lax use of terminology.

By themselves, neither a micrometer nor calipers are measuring devices.

Calipers or more correctly a pair of calipers are a pair of arms terminating in set faces or points. They may be set on a pivot like scissors or parallel and sliding like shown in the previous posts and may have a mechanism for locking them at a particular setting. There are even calipers, known as odd legs for use in tubes.

In use you either set them to a particular spacing or opening by measuresuremet against a standard such as a block or ruler or you set them physically across the object to be measured and then compare with a ruler.
More accurate reading can be made by means of a vernier. A special type as shown in the pevious posts incorporates a single stage vernier as shown

A micrometer is a device for converting rotary motion to linear motion by means of a leadscrew and nut. As you wind the screw through the nut the device is pulled along a linear path. The accuracy can be very high, much higher than for a single stage vernier. It depends upon how fine or coarse the screwthread is.
Notice I did not say it is just for measurement since the dials on a manual lathe or othr machine tool are also micrometers and they do not measure they pull the tool or workpiece along.

When used as a measuring device the simple micrometer lookslike the devices in the previous posts.
One thing they have omitted to mention directly is about the 'feel' they talk about.

On a quality micrometer, of this type, there is a ratchet or other mechanism so which releases the leadscrew when a certain clampinf force is achieved. The user knows this because he winds the screw until a click is heard and felt. At this point there is always the same clamping force between the faces.
That is why mike measurements should be highly reproducible.

You mention measuring a domed object.

Between high school and university I worked at a research establishment and one of my tasks was
"to measure the length of a brick" to 1/100,000" using a special device known as a floating micrometer.

Obviously the length of a brick is pretty vague, so we used special domed measuring points set into the ends of the brick a measured to these.
The actual micrometer rotated the brick so the maximum distance between the two measurement points was always achieved.

go well

 

[pantaz]

...
Micrometer calipers are more accurate than vernier calipers but I am guessing that there's a better reason behind the preference of micrometer calipers in measuring diameter.

In general usage, micrometer calipers are simply called, "micrometers", or "mics" (pronounced "mike" -- as in, "Hand me that one-inch mic"), and are assumed to be outside measuring. Micrometers typically measure only a 1"/25mm range per instrument. (Such as, 0-25mm, 25mm-50mm.) Also, a "25mm mic" refers to a 0-25mm micrometer, a "100mm mic" is a 75mm-100mm micrometer.

"Calipers" refers to dial/digital/vernier calipers. (See examples below.) Calipers typically measure a wide range, such as 0-150mm (0-6"), 0-300mm (0-12"), and up. (My largest one is a 600mm/24" vernier.) Also, calipers generally allow four different methods of measurement (outside, inside, depth, and step).

Micrometer

Digital Micrometer

Dial Calipers

Vernier Calipers

Digital Calipers

Micrometer frames are more rigid than calipers, permitting more precise and repeatable measurements. Outside micrometers commonly offer resolution to 0.0001"/0.001mm @ ±2µm accuracy*, while calipers typically provide only 0.001"/0.01mm resolution. Due to their less rigid construction, calipers are generally considered to provide accuracy not better than 0.001".

Accuracy is also highly influenced by the operator. Micrometers have built-in limiters (friction or ratchet thimbles) to insure consistent pressure is applied to the item being measured. A calipers' jaw pressure is entirely dependent upon the user. The operator must also be careful to place the jaws or anvils in proper contact with the measured item, and make sure all surfaces are clean and smooth.

* Published accuracy rating for http://www.mitutoyo.com/TerminalMerchandisingGroup.aspx?group=1095" outside mic.

 

[2milehi]

I'm sorry but I don't feel very comfortable with the explanation merely based on "feel." Can you provide a more rhetorical explanation?

Go measure the diameter of something with both and chime back in with feel.

To me when I have measured something correctly with a micrometer, I first get a minimum distance and then I pull the mic away. If I have measured "correctly", I will feel some drag on the mic. If I have measured too loose, the mic easily slides out. If I have measured too tightly I can't remove the mic from the item I am measuring.

You don't get this feel with a caliper.

Also micrometers come with standards so you can verify the "span" of the mic.

 

[Ranger Mike]

I have been involved in Metrology Since 1980. Both of these are used into " measure " parts..at best they are comparative measuring instruments. There is a litany of data and information telling how to properly use these items. at best you can hope for 0.0245 mm (.001") accuracy...which is dubious at best.

One needs a set of gage blocks to verify the accuracy of the instrument. The errors of the human operator are so numerous the highly quoted "Gage Makers Rule of TEN" came into effect pre WW2 causing much consternation to me to this day. Part temperature comes into play as well.

Practically you are measuring to +/- .002" with a caliper

Practically you are measuring to +/- .0001" with a 1 and 2 inch ratchet micrometer
Plain mics drop to +/- 0.001"
you may get better results with digital read outs but have not taken time to look it up

it goes tot +/- 0.002" when the mic gets over 6 inch and you really have to be up on the use of it to REPEAT.. more precise required further discussion.

ps - we have not even discussed calibration, accuracy, precision, resolution, uncertainty of measurement..

it goes on and on...